1. Field of the Invention
Embodiments of the invention relate to semiconductor devices for use in ignition devices for, for example, internal combustion engines.
2. Description of the Related Art
An ignition device for an internal combustion engine of a vehicle uses a semiconductor device installing a power semiconductor element for switching control of a primary current in an ignition coil. FIG. 12 shows an example of construction of a conventional, general semiconductor device for ignition of an internal combustion engine using a power semiconductor element of an insulated gate bipolar transistor (IGBT).
The semiconductor device for ignition shown in FIG. 12 includes an engine control unit (ECU) 501, which is an electronic control unit, a semiconductor integrated circuit (IC) for ignition (an ignition IC) 502, an ignition coil 503, a voltage source 504, and an ignition plug 505.
The ignition IC 502 includes an output stage IGBT 511 for ON/OFF controlling the primary current of the ignition coil 503, a sensing IGBT 512 having a common collector and a common gate with the output stage IGBT 511 and detecting sense current, a sensing resistor 513, a gate resistor 514, and a current control circuit 510 for controlling the collector current of the output stage IGBT 511. The ignition IC 502 has three terminals of a collector (C) terminal connecting to the ignition coil 503, an emitter (E) terminal connecting to the ground potential, and a gate (G) terminal connecting to the ECU 501.
Operation of the semiconductor device for ignition shown in FIG. 12 is described in the following. The ECU 501 delivers a signal for ON/OFF controlling the output stage IGBT 511 in the ignition IC 502 to the G terminal. When a signal at 5 V is given to the G terminal, the output IGBT 511 turns ON, and when a signal at 0 (zero) volts is given to the G terminal, the output stage IGBT 511 turns OFF.
Receiving an ON signal at the G terminal from the ECU 501, the output stage IGBT 511 of the ignition IC 502 turns ON and collector current Ic begins to flow from the voltage source 504 of 14 V, for example, through the primary coil 506 of the ignition coil 503, to the C terminal of the ignition IC 502. The Ic increases at a rate dl/dt that is determined by the inductance of the primary coil 506 and the applied voltage up to a current value, 13 A for example, controlled by the current control circuit 510 and remains at this current value.
Then, when an OFF signal is given from the ECU 501 to the G terminal, the output stage IGBT 511 of the ignition IC 502 turns OFF, decreasing the Ic rapidly. The rapid change of the Ic increases abruptly the voltage across the primary coil 506. At the same time, the voltage across the secondary coil 507 also rises to several tens of kilo volts, for example 30 kV. This high voltage is applied to the ignition plug 505, which discharges at voltages higher than about 10 kV.
In the case the ignition coil 503 or the ignition IC 502 might be damaged by overheating due to excessively long period of output of the ON signal from the ECU 1 or extraordinarily high temperature of the ignition IC 502, the Ic is shut down by operation of a self shut down circuit 533 installed in the current control circuit 510. However, abrupt shut down of the Ic may cause discharges of the ignition plug 505 at an unintended timing and damage to the engine. Accordingly, the Ic needs to be decreased at a dl/dt within a rate that does not cause erroneous discharge of the ignition plug 505.
FIG. 13 shows an example of construction of the current control circuit 510. This control circuit 510 is driven by a voltage between the G terminal and the E terminal, and includes a reference voltage circuit 531, a level shift circuits 532 and 534, a self shut down circuit 533, a comparator 535, and a metal oxide semiconductor field effect transistor (MOSFET) 536.
The reference voltage circuit 531 outputs a reference voltage Vref dividing, with a resistor 813 and a resistor 814, the voltage that is generated with a bias circuit composed of a depletion MOSFET (DepMOSFET) 811 and a MOSFET 812 that are connected in series and have a common gate terminal.
The level shift circuit 532 comprises: a bias circuit composed of a DepMOSFET 821 and a MOSFET 822 that are connected in series and have a common gate terminal, a MOSFET 823 composing a current mirror circuit together with the MOSFET 822, and a DepMOSFET 824 series connected to the MOSFET 823. Controlling the gate of the DepMOSFET 824 with the reference voltage Vref, the level shift circuit 532 generates and delivers a voltage that is a level-shifted voltage from the reference voltage Vref to a predetermined level of voltage.
The self shut down circuit 533 comprises: a bias circuit composed of a DepMOSFET 831 and a MOSFET 832 connected in series and having a common gate terminal, a MOSFET 833 composing a current mirror circuit together with the MOSFET 832, a MOSFET 834 connected in series to the MOSFET 833, and a capacitor 835. The MOSFET 834 is ON/OFF controlled with a self shut down signal SD generated by a means such as a timer circuit or a temperature detecting circuit (that are not shown in the figure). The MOSFET 834 is in an ON state in normal operation and in an OFF state in abnormal conditions. Setting the ON resistance of the MOSFET 834 being sufficiently smaller than the ON resistance of the MOSFET 833, the self shutdown circuit 533 delivers a voltage of level-shifted reference voltage Vref as it is in the case of normal operation, and in the abnormal conditions, decreases gradually the output voltage discharging the charges on the capacitor 835 by operating the MOSFET 833.
The level shift circuit 534 includes a bias circuit composed of a DepMOSFET 841 and a MOSFET 842 connected in series with a common gate terminal, a MOSFET 843 composing a current mirror circuit together with the MOSFET 842, and a DepMOSFET 844 connected in series to the MOSFET 843. The level shift circuit 534 controls the gate of the DepMOSFET 844 with the sense voltage Vsns detected by converting to a voltage value from a current value proportional to the Ic with the sense IGBT and the sense resistor 513 shown in FIG. 12. Thus, the level shift circuit 534 generates and delivers the sense voltage Vsns that has been level-shifted to a predetermined level of voltage.
The comparator 535 compares the output of the self shut down circuit 533 and the output of the level shift circuit 534 and ON/OFF controls the MOSFET 536 according to the comparison result. If the level-shifted sense voltage Vsns is lower than the level-shifted reference voltage Vref, the MOSFET 536 is in the OFF state, and if the level-shifted sense voltage Vsns is higher than the level-shifted reference voltage Vref, the MOSFET 536 is in the ON state.
Operation of the semiconductor device for ignition shown in FIG. 12 is described in the following with reference to FIGS. 14A and 14B. The FIGS. 14A and 14B show waveforms involved in the control of the current Ic, in which FIG. 14A shows the case in which self shut down is conducted after the collector current Ic has reached the limiting current value Ilim, and FIG. 14B shows the case in which self shut down is conducted without the IC reaching the Ilim. Although the reference voltage Vref and the sense voltage Vsns are level-shifted by the level-shift circuits 532 and 534, the remarks on these facts are omitted in the following description.
Referring to FIG. 14A, when an ON signal, for example 5V, is given from the ECU 501, the current Ic flows and the sense voltage Vsns rises. When the sense voltage Vsns reaches the reference voltage Vref at the time t1, the MOSFET 536 turns ON to decrease the gate voltage VGout of the output stage IGBT 511. After that, the control to maintain the relation Vref=Vsns is performed with the comparison circuit 535 in the period from t1 to t2. When the self shut down signal SD (indicated in FIG. 13) is given, the output of the self shut down circuit 533 gradually decreases from the reference voltage Vref, and the VGout also decreases holding the relationship Vref=Vsns in the period from t2 to t3. When the VGout reaches the threshold voltage Vth of the IGBT 511, for example 2V, the current Ic is completely shut down at the time t3.
While the output of the self shut down circuit 533 decreases to approximately zero volts according to the discharge of the capacitor 535, in order to keep the Ic in the completely shut down state, the relationship Vsns>Vref>0 needs to be held still in the period of Ic=0. The level shift circuit 534 is provided for this purpose, while the level shift circuit 532 is provided for adjusting characteristics with the sense side. After the sense voltage Vsns reaches the lower limit and remains at that value, the output voltage of the self shut down circuit 533 continues decreasing. Thus, the output voltage of the comparison circuit 535 rises abruptly and the gate voltage VGout falls abruptly.
Referring to FIG. 14B, in the case the voltage of the voltage source 504 is lowered and the current Ic does not reach the limiting current Ilim, when the self shut down signal SD is given to start self shut down operation and the condition Vref=Vsns is established, the gate voltage VGout falls abruptly at the time t4. This abrupt falling down of the gate voltage VGout generates oscillation in the current Ic and may cause erroneous ignition of the ignition plug 505.
Japanese Unexamined Patent Application Publication No. 2001-153012 (also referred to herein as “Patent Document 1”), for example, discloses a method to cope with the problem of erroneous ignition due to the Ic oscillation. The device of Patent Document 1 includes a series circuit of a voltage restraining IGBT and a jumping voltage-suppressing diode, the series circuit being connected in parallel with an output stage IGBT. When the collector voltage increases in operation of the output stage IGBT and exceeds a withstand voltage of the diode, the diode yields and an electric current flows through the voltage restraining IGBT to control the collector voltage at a constant voltage.
A device disclosed in Japanese Unexamined Patent Application Publication No. 2002-371945 (also referred to herein as “Patent Document 2”) comprises a voltage monitoring circuit for monitoring collector voltage of an output stage IGBT and a control current-adjusting circuit for limiting current flowing to the gate of the output stage IGBT according to the output from the voltage monitoring circuit. When a current limiting operation for the output stage IGBT begins and the collector voltage rises, the voltage monitoring circuit starts operation and the control current adjusting circuit increases the gate voltage of the output stage IGBT, suppressing the rise up of the collector voltage.
Japanese Unexamined Patent Application Publication No. 2008-045514 (also referred to herein as “Patent Document 3”) discloses a one-chip igniter having integrated components on a monolithic silicon substrate, the components including: an insulated gate bipolar transistor that performs shut off control to interrupt primary current flowing in an ignition coil according to an ignition signal delivered by an electronic control unit for an internal combustion engine to generate a high voltage at the ignition coil; a current limiting circuit that limits the primary current flowing in the ignition coil; and a circuit that performs soft shut off of the primary current when a ignition signal for a period of time longer than a predetermined time or abnormal heating is detected. The time for soft shut off is set in the rage of 17 to 135 ms so as to avoid occurrence of failure such as thermal breakdown of the chip and suppress harmful combustion such as back fire of the engine due to erroneous discharge at the plug in the process of shut off of the primary current through the ignition coil corresponding to detection of abnormality such as input of an ignition signal for a period of time longer than a predetermined time or detection of overheating of the chip.
Thus, Patent Document 3 discloses a method of setting a gradually decreasing time by providing a soft shut off circuit.
Japanese Unexamined Patent Application Publication No. 2006-037822 (also referred to herein as “Patent Document 4”) discloses an igniter detecting abnormality and performing self shut down, in which a rise up output of an abnormality detecting circuit is delivered, through an integration circuit composed of a diode and a capacitor, to the gate of a MOSFET for self shut down thereby gradually decreasing the gate voltage of an IGBT of a main current switching device. This means achieves a circuit having a time constant in the order of milli-seconds with a minimum circuit size and circuit area, and provides an igniter that performs self shut down without erroneous ignition upon abnormality detection.
Thus, Patent Document 4 discloses a method of setting a gradually decreasing time for the Ic by providing an integration circuit composed of a diode and a capacitor.
The conventional ignition devices for internal combustion engines described above can have one or more of the following problems.
The conventional semiconductor device for ignition shown in FIG. 12 may generate oscillation in the collector current Ic of the output stage IGBT in an operation time of the current control circuit or in an operation time of the self shut down circuit and cause erroneous ignition of the ignition plug.
The semiconductor devices for ignition disclosed in Patent Documents 1 and 2 prepare a countermeasure against the oscillation of collector current Ic of the output stage IGBT in operation of the current control circuit. However, the documents do not mention a countermeasure against oscillation of collector current Ic of the output stage IGBT in operation of the self shut down circuit, and thus have the similar problem to the one in the conventional semiconductor device for ignition shown in FIG. 12.
Likewise, the Patent Documents 3 and 4 make no mention about any countermeasure against oscillation of collector current Ic of the output stage IGBT in operation of the self shut down circuit, and have the same problem as the one in the conventional semiconductor device for ignition shown in FIG. 12. Thus, as described above, there exists certain shortcomings in the related art.